1. Field of the Invention
The present invention relates to a magnetic recording medium suited for use in recording and playback of signals, such as audio signals, video signals and data signals, and also to a method of producing such a magnetic recording medium.
2. Description of the Prior Art
In recent years, there has been a trend for reduction in the size and weight of audio and visual magnetic recording and playback devices, and the recording density has been increasing year by year. On the other hand, studies have been made for development of perpendicular magnetic recording systems in order to attain higher recording density. Studies have been made also for development of recording medium in which a thin film or layer of a magnetic material is formed on a substrate by a vapor deposition method, such as evaporation, sputtering or other known measure (such types of medium will be referred to as "vapor-deposited medium", hereinafter), besides the coating-type medium in which a magnetic layer is formed by coating a magnetic material in the form of a mixture with a binder.
FIG. 4 is a sectional view of a vapor-deposited medium. As will be seen from this Figure, the vapor-deposited medium has a substrate 1 made of, for example, polyethylene, a magnetic layer 2 consisting mainly of a magnetic material, such as Co, Cr, Ni or the like, and a protective layer 3. The surface of the substrate 1 remote from the magnetic layer 2 may have been subjected to a back coating treatment or a back surface treatment in order to improve the running performance.
A description will be made hereinafter of a as to the conventional vapor-deposited medium. In order to allow an efficient use of the magnetic characteristics of the materials, such as Co, Cr, Ni and Fe, the magnetic layer 2 is formed directly on the substrate 1 by a vapor deposition method, such as evaporation, sputtering or the like, rather than by coating the magnetic material in the form of a mixture with a binder. Therefore, the surface of the magnetic layer 2 is made only of a metal or a metal oxide, so that, when the recording medium is used in an audio tape recorder, video tape recorder (VTR), floppy fisk drive (FDD), etc. a large friction is caused between the magnetic head guide roller and the magnetic layer 2. This friction in turn causes wear or damage of the magnetic layer 2 and the magnetic head and the guide roller, as well as noises due to a stick slip. In order to avoid these problems, hitherto, it has been proposed to reduce the friction between the magnetic layer 2 and the stationary parts, such as the magnetic head and the guide roller, by covering the magnetic layer 2 with a protective film 3 having a lubricating effect or by suitably selecting the surface roughness of the magnetic layer 2.
Two types of methods have been proposed for the purpose of forming the protective film 3: namely, a method called "wet process" and a method called "dry process". In the wet process, a lubricant dissolved in a solvent is applied to the magnetic layer 2 and then the solvent is evaporated by, for example, heating, so as to leave a lubricant protective film 3 on the magnetic layer 2. On the other hand, in the dry process, the protective film 3 is formed by depositing a lubricant directly on the magnetic layer 2 by, for example, evaporation, sputtering, plasma CVD and so forth, without using any solvent. In the conventional dry process, the lubricant is in most cases an organic one and the thickness of the protective film usually ranges between 50 and 1000 .ANG., as reported in Japanese Patent Unexamined Publication Nos. 154643/1984, 35331/1985, 171029/1984, 213031/1984, 102330/1983 and 135440/1982. The conventional magnetic recording films with protective films, however, have suffered from the following disadvantges.
Referring first to the protective film formed by the wet process, the protective film tends to have island-type construction when the film thickness is small, because the film is formed by evaporating the solvent in which the lubricant is dissolved. This means that the magnetic layer 2 is partially uncovered so as to cause a large friction in the portions where the magnetic layer 2 is uncovered. Such a protective film is quite unsatisfactory from the view points of wear and corrosion resistance. The protective film formed by the wet process, therefore, should have a thickness not smaller than 200 .ANG., resulting in a large space loss. When the recording wavelength of information is 0.9 .mu.m, the playback loss is as large as about 1.6 dB when the thickness of the protective film is 200 .ANG.. The playback loss is much greater in a device of the perpendicular recording type which is intended for higher recording density. The wet process involves further disadvantages such as impossibility of application of some types of lubricant due to a repellent effect on the surface of the magnetic layer 2. In addition, when the film thickness is as large as 200 .ANG., the lubricant tends to come off and may be deposited on portions around the magnetic head, resulting in troubles such as clogging and drop out. The wet process also involves a risk of contamination by foreign matter and dust transferred from air and tools, resulting in defective products.
The dry process is superior and more promising than the wet process from the view point of contamination prevention and uniformity of the film. It is reported that the protective film formed by the dry process, mainly consisting of hydrocarbon and organic material containing hydrocarbon, has to have a thickness of 50 .ANG. or more from the view point of wear resistance. Such a large film thickness increases the tendency for the lubricant to come off and to be deposited to the area around the magnetic head, resulting in clogging and drop out, as in the case of the protective film formed by the wet process.
The conventional dry process also encounters problems such as difficulty in the control of the properties of the film and poor reproducibility. In addition, this process is not suited to mass-production because the film forming speed is much lower than that of the wet process.